Field of the Invention
The present invention relates to a method for diagnosing a degree of degradation of a catalyst that oxidizes or adsorbs an unburned hydrocarbon gas.
Description of the Background Art
Conventionally, for on-board diagnostics (OBD) of an exhaust gas cleaning system for vehicles, that is, the function of diagnosing whether a catalyst acts normally or not in the system, a large number of patent applications have been filed mainly by auto companies. Most of these patents relate to techniques using an exhaust gas temperature sensor, oxygen sensor, wide-range oxygen concentration sensor (λ sensor), NOx sensor, or PM sensor, and are targeted for three-way catalysts, oxidation catalysts, NOx storage catalysts, NOx selective reduction catalysts, and diesel particulate filters (DPFs) (for example, see Japanese Patent Application Laid-Open Nos. 2001-263048, 2005-240716, 2012-036860, 2012-241594, and 07-103039 (1995)).
Various types of hydrocarbon gas sensors (HC sensors) for measuring vehicle exhaust gases based on various principles have been researched and developed. A HC sensor of semiconductor type that is applicable to control of purification for NOx in a zeolite catalyst and is capable of selectively detecting a hydrocarbon (HC) having a large molecular weight is publicly known (for example, see Japanese Patent No. 2876793). HC sensors of various types have been widely known, such as catalytic combustion gas sensors, oxygen-concentration difference sensing gas sensors, limiting current gas sensors, and mixed-potential gas sensors, for which a large number of patent applications have been filed peaking around the year 2000.
Such HC sensors are, however, assumed mainly to be used in an inspection of the purification performance of exhaust gas purification devices (three-way catalyst (TWC), NOx storage catalyst (NSC)) mounted in a gasoline engine of a theoretical equivalent ratio (air excess ratio λ=1) combustion type or a lean-burn (λ>1) type, or used in control of injection amount for an unburned hydrocarbon in a diesel engine.
Entering the 2010s, regulations on exhaust gases have been tightened in the North America, and especially, OBD of oxidation catalysts for diesel engine vehicles will be made mandatory in the future. Specifically, oxidation catalysts for diesel engines have been required to undergo OBD targeted for nom methane hydro carbon (NMHC) in an excessive oxygen (O2) atmosphere that is an exhaust atmosphere of a diesel engine.
However, the publicly known techniques involving the use of sensors, as disclosed in Japanese Patent Application Laid-Open Nos. 2001-263048, 2005-240716, 2012-036860, 2012-241594, and 07-103039, cannot support such OBD or can only make an indirect diagnosis.
For example, disclosed in Japanese Patent Application Laid-Open No. 2001-263048 is a technique involving the use of a relationship that when the ability of converting (oxidizing or combusting) an unburned hydrocarbon in an oxidation catalyst decreases, exothermic energy will also decrease. In outline, a temperature difference AT is measured, which occurs at the time of fuel injection in exhaust gas temperature sensors disposed forward and reward (upstream and downstream) of an oxidation catalyst in an exhaust path, and the degree of degradation in the ability of converting (oxidizing or combusting) an unburned hydrocarbon in the oxidation catalyst is diagnosed indirectly from a measurement value.
In this method, however, a cause of error may be excessively large due to changes in the temperature and the flow rate of an exhaust gas when the gas sensors are practically used, and fuel consumption may inevitably degrade because a large amount of fuel injection is required for accelerating heat generation.
Disclosed in Japanese Patent Application Laid-Open No. 2005-240716 is a technique involving the use of a fact that when the ability of converting an unburned hydrocarbon in an oxidation catalyst decreases, the amount of oxygen consumed during the combustion of oxygen changes. In outline, on the basis of a difference Δλ between output values λF and λR of two wide-range oxygen concentration sensors (λ sensors) disposed forward and reward of an oxidation catalyst in an exhaust path, or a difference between output values (electromotive force values) of two oxygen sensors, an amount of oxygen consumed in an oxidation catalyst is measured, and the degree of degradation in the ability of converting an unburned hydrocarbon on the oxidation catalyst is diagnosed indirectly from a change in the measured value.
However, the concentration of oxygen in a diesel exhaust that is an excessive O2 atmosphere is approximately 10% (=100000 ppm), whereas the amount (concentration) of hydrocarbon that is converted (oxidized or combusted) by an oxidation catalyst is normally on the order of several hundreds of ppm, and the amount (concentration) of oxygen consumed when such a trace amount of hydrocarbon is burned is no more than several hundreds of ppm. This means that the diagnosis of the degradation of an oxidation catalyst with an air-fuel ratio sensor or oxygen sensor requires an accurate calculation of Δλ or a difference in electromotive force corresponding to a ppm-order change in the amount of oxygen consumed, but the air-fuel sensors and oxygen sensors originally cannot achieve such accuracy in measurements.
Disclosed in Japanese Patent Application Laid-Open No.2012-036860 is a technique for disposing a NOx sensor downstream of an oxidation catalyst that oxidizes NO into NO2 in an exhaust path and determining a degree of degradation of the oxidation catalyst on the basis of a predetermined map and an output value (electromotive force value) of the NOx sensor.
However, even if such a technique can diagnose the ability of oxidizing NO of an oxidation catalyst, the result of this diagnosis cannot be applied to diagnosis of the ability of converting (oxidizing or combusting) an unburned hydrocarbon. This is because the functions of a precious metal catalyst and a storage agent vary for the types of gases (e.g., HC, CO, NO), and accordingly, the relationship between the temperature of an exhaust gas and a conversion (oxidization or combustion) rate also differs in the respective gases, and no specific correlation is found therebetween.
Additionally, because an estimated value is used as a NOx value for the exhaust immediately after a discharge of an engine or factors except for an engine speed and an engine load are not taken into account in setting of such an estimated value, it is conceivable that the accuracy of estimation will degrade remarkably depending on usage condition.
Disclosed in Japanese Patent Application Laid-Open No. 2012-241594 is a method for disposing an exhaust gas temperature sensor and a λ sensor forward and reward of an oxidation catalyst and diagnosing a degree of degradation of the oxidation catalyst on the basis of an amount of required oxygen obtained from an estimated value of a HC storage capability during a normal activity of the oxidation catalyst and an estimated value of an amount of oxygen actually consumed, which is an amount of oxygen actually consumed by the oxidation catalyst.
Such a technique, however, merely makes a diagnosis based on an estimated value, and is inevitably affected by errors of signals from the respective sensors, leading to low diagnosis accuracy.
Disclosed in Japanese Patent Application Laid-Open No. 07-103039 is a system whose diagnosis target is a TWC or NSC of a gasoline engine. Japanese Patent Application Laid-Open No. 07-103039 discloses nothing about the diagnosis of an oxidation catalyst in diesel exhaust that is an excessive O2 state.